Communications devices, particularly those that implement digital subscriber line (DSL) technologies (e.g., T1 and xDSL, including SDSL, HDSL, HDSL2, SHDSL, ADSL, G.shdsl, etc.), transmit high-speed data using analog signals over telephone connections, which are typically copper wire pairs. The transmission of high-speed data over copper wires may be impaired by impulse noise in the transmission path. Coding of data is used to minimize the adverse affects of impulse noise.
Impulse noise can originate from many sources. For example, Plain Old Telephone Service (POTS) ringers introduce broadband Near End Cross Talk (NEXT) impulses. Drop or entrance cable and premises wiring are large contributors to impulse noise. Impulse noise events are likely correlated over several symbol (or baud) periods of the DSL modulation. Such correlated noise can corrupt several consecutive symbols. Correlated noise may also be due to transient disturbances such as gain or timing phase changes in the channel. The corruption from transient disturbances may last until corrected by automatic gain control or timing acquisition circuits in the receiver.
Coding in Single-pair High-bit-rate DSL (SHDSL) and High-bit-rate Digital Subscriber Line 2 (HDSL2) may be accomplished with convolutional encoders and precoders. Convolutional encoders are used to implement Forward Error Correction (FEC) coding techniques (also known as convolutional coding). Typically, in forward error correction coding the transmitter encodes data by adding redundant bits systematically to the data bits so that, normally, only predetermined transitions from one sequential group of bits (corresponding to a symbol, or baud) to another are allowed. There is an inherent correlation between these redundant bits over consecutive symbols. At the receiver, each symbol is tentatively decoded and then analyzed based on past history, and the decoded bits are corrected, if necessary.
One well-known and widely accepted error coding technique is Trellis Coded Modulation (TCM), which is a form of convolutional coding that is optimized according to a specific modulation scheme. A TCM encoder is situated at the transmitter, and a TCM decoder is situated at the receiver. TCM is highly desirable since it combines the operations of modulation and error coding to provide effective error control coding without sacrificing power and bandwidth efficiency. TCM essentially averages the noise over more than one of the symbols. However, noise that is correlated over the constraint length of the trellis code will degrade performance of the decoder. In many cases, correlated noise causes the trellis decoder to perform worse than if the receiver employed no trellis coding at all.
In addition to the TCM encoder, precoders are designed to compensate for some correlated noise in the signal path. However, precoders can also break down if the impulse noise is correlated over several symbol periods. The correlated noise may be due to linear distortion in the transmit path of the noise source. The precoder may be of little benefit for linear distortion in the transmit path.
One method of addressing correlated noise problems is to interleave symbols. However, the generalized convolutional encoders employed with SHDSL and HDSL2 have limited interleaving capability. Thus, a need exists in the industry to overcome the aforementioned deficiencies and inadequacies.
The various DSL technologies employ a variety of line coding, e.g. 2 Binary, 1 Quaternary (2B1Q), Quadrature Amplitude and Phase modulation (QAM), Carrierless Amplitude and Phase (CAP) modulation, and Discrete Multitone (DMT). SHDSL and HDSL2 use Pulse Amplitude Modulation (PAM) coding to produce signals to be transported from transmitter to receiver.
As examples of the prior art described above, U.S. Pat. No. 5,659,578 to Alamouti et al., and U.S. Pat. No. 4,677,625 to Betts et al., describe the concept of TCM. The '625 Patent describes a distributed trellis encoder that can be used to spread symbols associated with a data stream over time across successive symbol periods, i.e. interleaving. The '625 Patent employs a plurality of trellis encoders to interleave the symbols. In addition, the ITU (International Telecommunication Union) has compiled a draft specification titled “G.991.2 (ex G.shdsl)—Single-Pair High Speed Digital Subscriber Line (SHDSL) Transceivers” (G.991.2). G.991.2 (ex G.shdsl) is available from the ITU, Geneva, Switzerland, at http://www.itu.int. Draft G.shdsl describes a system and method for providing SHDSL service. U.S. Pat. No. 5,659,578; U.S. Pat. No. 4,677,625; and G.991.2 are incorporated herein by reference.
DSL technologies are still in a state of infancy and are being improved over time by engineers and designers. The industry still needs ways to further enhance DSL communications and, in particular, ways to minimize the adverse effects of impulse noise. Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.